1. Field of the Invention
The present invention relates to use of a specific class of diaminoalcohols in admixture with an amino acid as cosolvent therefor in an aqueous absorbing solution for "hot pot" type acid gas scrubbing processes.
2. Description of Related Patents
Recently, it was shown in U.S. Pat. No. 4,112,050 that sterically hindered amines are superior to diethanolamine (DEA) as promoters for alkaline salts in the "hot pot" acid gas scrubbing process. U.S. Pat. No. 4,094,957 describes an improvement to this process whereby amino acids, particularly sterically hindered amino acids, serve to prevent phase separation of the aqueous solution containing sterically hindered amines at high temperatures and low fractional conversions during the acid gas scrubbing process.
While combinations of sterically hindered diamines such as N-cyclohexyl propanediamine (CHPD) with a cosolvent such as pipecolinic acid represent preferred activator systems in U.S. Pat. No. 4,094,957 for promoting hot carbonate CO.sub.2 scrubbing operations, there are certain disadvantages associated therewith. One difficulty is that sterically hindered primary-secondary diamines such as CHPD are unstable in the presence of CO.sub.2 and tend to enter into undesirable intramolecular condensation reactions therewith to form a cyclic urea precipitate, which reactions are catalyzed by any H.sub.2 S present in the gaseous system. The side reaction of CHPD with CO.sub.2 can be represented as follows: ##STR1## Not only is the diamine consumed by this reaction, but the insoluble cyclic urea must be removed from the system to avoid congestion of the plant operation. Another drawback is that many sterically hindered diamines are highly volatile, resulting in substantial loss thereof during the scrubbing process. This volatility problem is not unique to unstable diamines but is also encountered with many other diamines which are stable in the presence of CO.sub.2 and are effective in enhancing the rate of CO.sub.2 scrubbing.